The present invention pertains to manganese-iron base and manganese-chromium-iron base austenitic alloys. It is especially concerned with those alloys having resistance to neutron irradiation induced swelling and low post irradiation residual radioactivity (i.e. low activation).
Over the years, a number of austenitic, nickel-chromium-iron base, alloys and ferritic, chromium-iron base, alloys have been studied and developed for use in the high temperature, high energy neutron (0.1 MeV to 1.0 MeV) environment encountered in a liquid metal fast breeder reactor (LMFBR)--a fission reactor. One of the prime objectives of the LMFBR alloy development program has been to develop alloys, which are swelling resistant and have the required irradiation mechanical properties for use as fuel cladding and/or use as ducts. The fuel cladding will see service in contact with flowing liquid sodium and have a surface temperature of about 400.degree. C. (.apprxeq.750.degree. F.) to 650.degree. C. (.apprxeq.1200.degree. F.). A duct surrounds each bundle of fuel pins and sees service at about 380.degree. C. (.apprxeq.715.degree. F.) to 550.degree. C. (.apprxeq.1020.degree. F.). These components will be exposed at the aforementioned elevated temperatures to neutron fluxes on the order of 10.sup.15 n/cm.sup.2.s (E&gt;0.1 MeV), and should be capable of performing adequately to fluences on the order of 2 to 3.times.10.sup.23 n/cm.sup.2 (E&gt;0.1 MeV).
The Fe-Ni-Cr austenitic alloys being evaluated include the austenitic stainless steels described in U.S. Pat. Nos. 4,158,606; 4,407,673; and 4,421,572. In addition, Fe-Ni-Cr austenitic superalloys are being evaluated and have included those described in U.S. Pat. Nos. 4,040,876; 4,129,462; 4,172,742; 4,225,364; 4,359,349; 4,359,350; 4,377,553; and U.S. patent application Ser. No. 370,438 filed Apr. 21, 1982.
The foregoing efforts have been aimed at providing materials for the LMFBR environment. The fusion reactor, or CTR (Controlled Thermonuclear Reactor), also requires development of structural materials for use in its neutron irradiation environment. While a large number of, differing, fusion reaction designs have been proposed, a common requirement is the need for a low swelling, low activation material having good irradiation mechanical properties for use as a "first wall" material. The first wall forms a vacuum chamber which will hold the hot (up to 10.sup.8 .degree. K. in its interior) plasma in which the fusion reaction takes place. The plasma side of the first wall may be coated with a protective material such as graphite or silicon carbide. The opposite side of the first wall may be in contact with a fluid medium such as helium, water, liquid lithium or a liquid lithium-lead alloy, for example. Examples of some of the Fusion Reactor designs, and first wall materials being considered, are provided in J. T. Adrian Roberts, "Structural Materials in Nuclear Power Systems" (published in 1981 by Plenum Press New York) at pages 1-12, 279-319, which are hereby incorporated by reference.
As in the LMFBR, the environment in which the fusion reactor first wall material will operate is an elevated temperature neutron irradiation environment. However, this fusion environment will significantly differ from the LMFBR environment in that the energy of the source neutrons from a D-T (deuterium-tritium) fusion reaction is expected to be on the order of about 14 MeV compared to the 0.1 to 1.0 MeV mean neutron energies in the LMFBR fission process. This difference is important in that some of the critical alloying elements in the previously discussed austenitic alloys have (n,p) and (n,.alpha.) transmutation reactions which are activated by neutrons having energies greater than about 10 MeV, producing radioactive products with long half-lives. This leads to a nuclear waste handling and long term storage problem.
A goal of the fusion reactor first wall alloy development program is to select or develop an alloy which will not only have the needed swelling resistance and mechanical properties, but will also have relatively low residual radioactivity (i.e. "low activation"), allowing relatively short burial times (e.g. 80 to 100 yrs.) prior to reprocessing of the material. To meet this low activation goal, significant restrictions on the amount of the following elements in first wall materials have been proposed: Niobium &lt;3 ppm; Mo &lt;30 ppm; Cu &lt;0.1 wt %; N &lt;0.3 wt %; and Ni &lt;0.9 wt %. The restriction on nickel, and in most cases molybdenum, would eliminate the aforementioned austenitic Ni-Cr-Fe base alloys, developed for the LMFBR, from consideration as first wall materials. Other elements which may require significant restriction are: Ag, Bi, Tb, Ir, Eu and Ba.
Since 1882, when the first austenitic manganese steel was developed by Sir Robert Hadfield, a number of additional austenitic manganese steels have been developed. For the most part, these steels have high toughness, high ductility, a high work hardening coefficient, and good abrasion resistance. Typically, these alloys have been used in commercial applications requiring high toughness and high wear resistance. Some of the nominal commercial compositions, processing, and uses of austenitic manganese steel are listed in: R. B. Ross, "Metallic Materials Specification Handbook", (1980, E. & F. N. Spon Ltd.) at pages 369, 370, 579-582; and ASM (American Society for Metals), "Metals Handbook Ninth Edition, Volume 3--Properties and Selection: Stainless Steels, Tool Materials and Special Purpose Metals" (1980, ASM), at pages 568 to 588. The aforementioned pages from Ross and ASM are hereby incorporated by reference.